This invention is directed to the field of solventless coatings and, more particularly to such coatings that can be cured upon exposure to radiant energy.
The coating compositions used in standard coating operations often contain volatile organic compounds (VOCs), solvents, that are driven off during drying and curing operations. xe2x80x9cEnd-of-pipexe2x80x9d pollution control equipment is frequently used to capture such emissions in order to prevent them from becoming undesirable air pollutants.
The radiant energy curable coating compositions of this invention include compositions that cure upon exposure to electromagnetic radiation such as ultraviolet light or radio frequency energy or to ionizing radiation from an electron beam. When coating compositions are to be cured by ultraviolet light, photo initiators are added to the composition. When the radiant energy is a radio frequency field, initiators such as peroxides that decompose with heat generated by the radio frequency field are added to the composition.
These radiant energy curable coating compositions have been found to be particularly efficacious in providing decorative and corrosion resistant coatings on metals. Of interest is the use of these coatings on metal sheet that is processed in a continuous manner and wound into a coil.
U.S. Pat. No. 6,025,024 (Heindrichs, et al.) discloses a metal coil coating process that includes the steps of applying a wet layer of coating on a substrate and heating the coated substrate to dry the coating. The coatings used by Heindrichs et al. all contain substantial amounts of volatile organic compounds, solvents, in order to reduce viscosity for coating application. These coatings have a specified viscosity and are applied to a metal substrate moving at speeds of at least 60 meters per minute. The wet coating is passed under a roll having a raised pattern. The coatings employed can be based upon polyester-urethanes, epoxies, polyesters, polyurethanes, silicone-polyesters, polyvinyl chloride organosols, or polyvinylidene fluoride homopolymers and copolymers. The ""024 patent is incorporated herein by reference.
U.S. Pat. No. 6,004,629 (Madigan) discloses a sheet metal coil coating process. The sheet metal can be coated, dried and cured at speeds of 180 to 245 meters per minute (Col. 3, line 5). Such coatings can be cured by exposure to ionizing radiation from an electron beam. The ""629 patent is incorporated herein by reference.
U.S. Pat. No. 5,726,216 (Janke, et al.) discloses epoxy resins systems containing various polymers that must contain diaryliodonium type initiators so that they can be cured upon exposure to ionizing radiation. Such curing reactions proceed by cationic mechanisms and pose production challenges in high speed coating operations. Cationic curing entails an extended propagation step between initiation and final or complete cure. In high speed coil coating operations, such cationic systems could result in coil wound upon itself before the curing reaction had been completed. This would result in a blocked, useless roll of metal. The ""216 patent is incorporated herein by reference.
The coating compositions of the present invention comprise 1,2-polybutadienes that have a number average molecular weight (Mn) of about 500 to about 50,000 Daltons. In a preferred embodiment, the 1,2-polybutadienes have molecular weights of about 1,000 to about 5,000 Daltons. The 1,2-polybutadienes refer to butadiene oligomers that are polymerized primarily in a manner that yields pendant vinyl groups because only one of the two double bonds in the butadiene monomer is involved in the polymerization process.
The homogeneous blends of the present invention are achieved by enhancing the solubility of acrylated bis-phenol-A derivatives with the 1,2-polybutadiene oligomers by the use of a mutually compatible reactive material that co-cures into the composition. Such reactive materials can be members selected from the group consisting of long-chain aliphatic monofunctional and multifunctional acrylates and methacrylates, long-chain polyoxyalkylene monofunctional and multifunctional acrylates and methacrylates and reactive monomers and compounds substituted with such alkyl and/or alkoxy segments. Such blends do not exhibit phase separation on standing. When applied as coatings and cured using radiant energy, such blends produce coatings that are hard and extensible, exhibiting the needed impact resistance and ductility for bending or forming precoated metal as is commonly practiced when using metal coil stock.